The present invention relates to the field of data communications. Merely by way of example, the present invention is illustrated as a method and related apparatus for converting dual 4-wire digital data services for transmission over a single twisted pair telephone line.
Techniques for the transmission of multiple voice or data signals over a 4-wire loop are well known in the telecommunications industry. Conventional digital data services (DDS) over a 4-wire local loop include digital services at transmission rates of 2.4 kilo-bits/sec. (kbps), 4.8 kbps, 9.6 kbps, 19.2 kbps, and 56 kbps. Business subscribers often rely upon the conventional DDSs for use with electronic data processing equipment such as telephones, computers, facsimile machines, and the like. As the subscribers increase their use of such electronic data processing equipment, the need for DDSs also increases accordingly. However, it often is difficult, costly, and time consuming to add an additional 4-wire local loop onto a customer premises, thereby making it difficult to increase the amount of data communication services.
For example, the conventional DDS often needs to transmit digital data over "clean" telephone lines, that is, telephone lines that substantially have no abrupt changes in impedance values from wire gauge changes, bridge taps, coils, and the like. In particular, telephone companies often find it necessary to condition pre-existing telephone lines to remove such wire gauge changes, bridge taps, coils, and the like before implementing the conventional DDS onto the pre-existing telephone lines. In fact, telephone companies refer to this type of conditioning as a class C3 data conditioning technique. The C3 data conditioning technique often takes up to two weeks or even more for a typical 10-12 kilo-feet 4-wire local loop.
Another limitation with the use of the conventional 4-wire DDS is the need for repeaters when the loop exceeds 10-12 kilo-feet. Some telephone companies even install a repeater at the customer premises if signal losses fall below a pre-determined sound level such as 32 dBs using 28 kHz, 48 kHz, and 80 kHz test signals. The use of the repeater at the customer premises occurs because certain data service unit/channel service units (DSU/CSU) operate ineffectively at low signal levels. The conventional DDS relies upon a typical bipolar signal at a transmission rate of up to 56 kbps which often cannot flow effectively over a line span of 10-12 kilo-feet without the use of a repeater. The use of such repeater increases the cost of the conventional DDS for transmission lengths greater than 10-12 kilo-feet, and also decreases the efficiency and reliability of the conventional DDS by introducing additional elements into the telephone line span.
Still further, the conventional 4-wire DDS service requires the use of four wires, typically two twisted pair lines. Accordingly, it is often difficult, time consuming, and labor intensive to, for example, temporarily relocate equipment such as video tele-conference equipment and the like from location to location due to the limited availability of the two twisted pair lines at a typical customer premises. In addition, the use of two twisted pair lines for providing conventional DDS is generally an inefficient deployment of copper, which is a limited telephone company resource.
From the above, it is seen that a method and apparatus for providing an increase in data services from a telephone company central office to a customer premises over pre-existing wires is often desired.